*Where \f$x_c^{src}\f$ is the centre of the cell where the multiplole are considered,\f$x_c^{target}\f$ is the centre of the current celle. The celle where we compute the local expansion.
* operator do not use symmetries.
*
* Formula : \f[ L_{\mathbf{n}}^{c} = \frac{|n|!}{n! n!}